1. Field of Invention
This invention relates to digital video tape recorders (VTRs), and more particularly is directed to a tracking control system for a VTR.
2. Description of the Prior Art
In conventional helical-scan VTRs it is known to record a tracking control signal in a longitudinal track on the magnetic tape. A VTR of this type is semi-schematically shown in FIG. 7, in which reference numeral 30 generally designates the VTR.
VTR 30 includes a rotary drum 31 about which a magnetic recording tape 32 is helically wrapped. Tape 32 is advanced by a capstan 33, and is supplied from and taken up by, respectively, reels 34 and 35 of a tape cassette 36. A magnetic recording and reproducing head 37 is mounted on rotary drum 31 and scans tape 32 as tape 32 is advanced. During recording, magnetic head 37 forms inclined tracks in which the video signals are recorded on tape 32. During reproduction, magnetic head 37 scans the previously recorded tracks to reproduce the video signals. A fixed head 38 is used for recording and erasing cue and control signals, and the like, in a longitudinal track on tape 32 that is separate from the inclined video signal recording tracks.
FIG. 8 shows a signal recording format used in VTR 30 of FIG. 7. In FIG. 8, arrow A indicates the direction in which tape 32 is advanced. Inclined tracks T are scanned by magnetic head 37 in a direction indicated by arrow H. Control signals CTL are recorded by head 38 in a longitudinal track of tape 32. Tracking control is performed on the basis of an interval between the time at which a reference signal (not shown) is detected and the time at which the control signal CTL is detected.
The time interval between detection of the reference signal and detection of the control signal CTL is a function of the distance D between the beginning of track T and the location of control signal CTL. However, the distance D is subject to variation due to such factors as deterioration of tape 32 on account of age, variations in the tension in tape 32 during recording, and inconsistency in the performance of the tape transport mechanism. Since the conventional tracking control technique is based on maintaining a constant interval between detection of the reference and control signals, variations in the distance D may result in tracking control errors. A further disadvantage of the conventional track following system used in the VTR of FIG. 7 is the need to provide the fixed head 38.
Another known tracking control technique is referred to as the "ATF" system and is used in the 8 mm format VTR. In the ATF system, pilot signals are recorded in the helical tracks. Tracking control is based on the relative strength of cross-talk signals produced from pilot signals in adjacent tracks on either side of a track that is being scanned. In particular, the system adjusts the head scanning locus so as to equalize cross-talk signals received from the respective adjacent tracks. However, variations in the recording and reproducing efficiency of the signal recording and reproducing head again may result in tracking errors.